In order to increase the capacity on an optical transmission line (point-to-point connection) or in an optical network (multipoint-to-multipoint connection) there are today several known techniques. One of these comprises utilizing wavelength division multiplexing (WDM) of transmission channels to enhance the utilization of bandwidth in a network, which, however, requires access to devices capable of multiplexing and demultiplexing transmission channels, which are transmitted at different so-called optical carrier wavelength in the network.
One kind of multiplexing is so-called add/drop multiplexing, which means that a wavelength channel or channel group either is added to a multiplex (add function) or dropped from a multiplex (drop function). Known devices for add/drop multiplexing comprises i.a. the following.
A multiplexer device provided with Bragg gratings and based on a MZI-structure (MZI, Mach-Zehnder interferometer) is described in Low Crosstalk Optical Add-Drop Multiplexer based on a Planar Silica-On-Silicon Mach-Zehnder Interferometer with UV-induced Bragg Gratings and UV-Trimming, J.-M. Jouanno et al, Tech. Dig. Bragg Gratings, Photosensitivity, and Poling in Glass Fibers and Wavequides; Applications and Fundamentals, OSA, 1997, Williamsburg, Va. pages 259-261. In principle a complete add/drop multiplexer may be achieved by such a device, which comprises two directional couplers interconnected by interferometer legs, a co-called MZI-structure, even if it is most likely in practice that two separate devices are required to achieve complete add/drop functionality. Alternatively, two Bragg gratings having equal Bragg wavelengths may be cascaded in each Mach-Zehnder leg. Further, coupling to so-called cladding modes in the grating structures may occur, which leads to deteriorated performance of the device, particularly for channels whose carrier wavelengths are shorter than the Bragg wavelength.
A wavelength selective tunable device, denoted MM/MZI demultiplexer, (MM/MZI, multimode interference Mach-Zehnder interferometer) can be used for wavelength selective switching, see for instance the publication A new type of tunable demultiplexer using a multi-leg Mach-Zehnder interferometer, J.-P. Weber et al, Proc. ECIO '97 EthE5, Stockholm, pages 272-275, 1997. Cascading of two such devices may result in a complete tunable add/drop multiplexer. Such a multiplexer, however, has a very narrow region where the crosstalk performance is good (low crosstalk), which in a straightly principle way is possible to consider, but in such instance very complexed interference circuits are required to achieve non-linear phase response in the Mach-Zehnder legs of the multiplexer. Further, interference problems may occur for transmitted channels when two MMIMZI devices are cascaded.
A wavelength selective device based on an MMIMIBg structure (MMIMIBg, Bragg grating assisted multimode interference Michelson interferometer), which provides for completely individual switching, is depicted in Bragg grating assisted MMIMI coupler for wavelength selective switching, T. Augustsson, Electron. Lett., volume 34(35), pages 2416-2418, 1998. Even if the theory of the technique indicates low crosstalk, e.g. process dependent tolerance variation effects may come to increase the crosstalk. The MMIMIBg device is probably particularly sensitive for variable losses as regards crosstalk since it is based on reflection in long Michelson interferometer legs. Possibly, two separate devices are required to achieve complete add/drop functionality. Further, it has not yet been experimentally shown that such a device having acceptable performance may be realized.
An MMI based device provided with Bragg gratings is shown in the paper Bragg Grating-Assisted MMI coupler for Add-Drop Multiplexing, T. Augustsson, J. Lighwave Tehnol., volume 16(8), pages 1517-1522, 1998. However, it is difficult to achieve such a device which can handle a channel separation below 400 GHz with good filter performance.
Problems of the above mentioned known techniques comprise thus a long propagation distance for the light through the respective device, which gives rise to power losses and risk of instabilities. Further, known techniques may involve severe problems as regards channel crosstalk and interference effects. The devices, which have good performance, are relatively complicated and thus relatively difficult and costly to realize.